Make sure MetadataImporterPluginFactory has external linkage

Otherwise, in a LO built with hidden visibility (as gcc terminology,
not ELF, Mac OS X does not use ELF), Spotlight won't be able to use
our importer.

Change-Id: Ie19082f9f14e4cc75ae6f7583031b929b7b75330

extensions/source/macosx/spotlight/main.m

@@ -212,6 +212,7 @@ ULONG MetadataImporterPluginRelease(void *thisInstance)
 // -----------------------------------------------------------------------------
 //    Implementation of the factory function for this type.
 //
+__attribute__ ((visibility("default")))
 void *
 MetadataImporterPluginFactory(CFAllocatorRef allocator, CFUUIDRef typeID)
 {